We have just switch from ATM to MPLS circuits for all of our remote sites. We use ISDN for fail over when the Primary T1 circuit goes down. Problem: ISDN will not come up when the primary circuit goes down, however we can manually force ISDN up, for approx. 15 minutes, after which time the ISDN will go back down. Has anyone seen this issue before and what steps can I take to trouble shoot and or resolve this issue?

Thanks for any help I can get to help solve this problem.